A bolt is a type of fastener that is inserted through a pre-existing hole in material. A bolt is typically threaded and is secured with a threaded nut. There are bolts that use force-fit nuts, which do not require threads. A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a screw or nut.
When mechanical components are fastened together, using a bolt, washer, and nut, the washer and bolt remain in place during placement and turning of the nut. Keeping these parts in place by hand can be time consuming and difficult, especially in confined spaces.